DESCRIPTION: This proposal is aimed at obtaining three-dimensional structural information about the process of recombination catalyzed by the lambda integrase (Int) protein. Int catalyzes the integration of the bacteriophages lambda into the bacterial host, as well as the excision of the genome during lysis. The reaction has some similarity to that catalyzed by Type I topoisomerases, in that a covalent linkage is formed between a tyrosine residue in the protein and the phosphate backbone of DNA. Building on previous work from his laboratory, Dr. Ellenberger proposes to study by x-ray crystallography the structures of lambda integrase bound to pieces of DNA corresponding to various stages in the reaction cycle. Crystals have been obtained of a dimer of lambda integrase bound to uncleaved DNA through the tyrosine linkage. This structure will resolve issues relating to whether the tyrosine residue of one integrase protein catalyzes bond cleavage within the active site of the same molecule, or whether it does so in that of the neighboring molecule. Interest in this issue arises because of a fundamental interest in understanding how cooperativity is utilized in driving DNA cleavage and religation. Further aims of the proposal include the determination of the structure of lambda integrase complexed with a four way Holliday Junction, as well as a very large (in terms of crystallography) DNA hairpin structure stabilized by the DNA bending protein known as IHF.